Cultured skin fibroblasts from normal and diabetic subjects are studied during their life span in vitro. Senescence is monitored by mean population doublings, sequential 5-day growth curves, plating efficiency, timed plating efficiency, and DNA synthesis. Parallel studies of collagen synthesis are run using incorporation of 14C proline followed by SDS-gel electrophoresis and amino acid analysis. Juvenile diabetic subjects are divided into groups as follows: newly diagnosed, 1-5 years, 5-10 years, and 10 plus years duration; all are studied along with age-matched controls. Results obtained so far suggest that diabetic cells age faster in culture than normal cells. This increased rate of aging appears to be affected by the duration of diabetes in vivo before in vitro studies are begun because cells from diabetics with disease of 10 plus years' duration age faster than cells from subjects with recent onset of the disease. Plating efficiency and sequential growth curves are a sensitive index of cell aging in culture. Diabetic cells also incorporate more 14C proline into secreted collagen proteins than normal cells.